1. Field of the Invention
The present invention relates to a micromirror actuator and, more particularly, to a micromirror actuator, which can be driven by attractive electrostatic forces and can have a wide range of driving angles with the use of a low driving voltage.
This application is based on Korean Patent Application No. 2001-36092 filed on Jun. 23, 2001, the disclosure of which is incorporated herein by reference in its entirety.
2. Description of the Related Art
Micromirrors are generally used in various fields including an optical communication field. For example, micro-optical cross connects (MOXCs) are devices for selecting an optical path and thus allowing an optical signal to be transmitted from an input terminal to a predetermined output terminal, and micromirrors are the most important components of MOXC.
Micromirror actuators, which can maintain a micromirror at a predetermined angle, have been manufactured by using many different methods and having various structures. FIGS. 1A and 1B are views illustrating a conventional micromirror actuator using attractive electrostatic forces and including an elastic element, such as a torsion spring. Referring to FIGS. 1A and 1B, a micromirror 14 is formed horizontally, over a substrate 11 with a spring unit 13 supported by a protrusion 12 formed on the substrate 11. The micromirror 14 is formed to be capable of rotating, and a lower electrode 15 is formed under the micromirror 14. If external voltage is applied to the micromirror 14 and the lower electrode 15, attractive electrostatic forces are generated between the micromirror 14 and the lower electrode 15, as shown in FIG. 1B. Due to the electrostatic forces, the micromirror 14 supported by the spring unit 13 inclines over the substrate 11 at a predetermined angle.
The range, in which the driving angle of the micromirror actuator driven by attractive electrostatic forces can be controlled by external voltage, is strictly restricted due to the special characteristics of the method of driving the micromirror actuator. In other words, if a voltage not less than a predetermined level, i.e., threshold voltage, is applied between the spring unit 13 and the micromirror 14, attractive electrostatic force generated by the applied voltage is always stronger than the elastic restoring force of the spring unit 13. Thus, the distance between the micromirror 14 and the lower electrode 15 becomes shorter. This can be described more thoroughly with reference to Equation (1).                     F        =                              ϵ            ⁢                          xe2x80x83                        ⁢                          AV              2                                            d            2                                              (        1        )            
In Equation (1), F represents an attractive electrostatic force, xcex5 represents a dielectric constant, A represents the area of an electrode, V represents a potential difference, and d represents the distance between the electrodes. In general, the intensity of the attractive electrostatic force acting between the electrodes is inversely proportional to the square of the distance between the electrodes but is proportional to the square of a voltage applied to the electrodes. Accordingly, as the distance d between the electrodes decreases, the influence of the voltage applied to the electrodes on the attractive electrostatic force between the electrodes increases. In addition, the range, in which the driving angle of a micromirror can be controlled by the voltage applied to the electrodes, becomes very sensitive to a potential difference between the electrodes.
On the other hand, as the distance d between the electrodes increases, the influence of the voltage applied to the electrodes on the attractive electrostatic force between the electrodes, decreases, and the range, in which the driving angle of a micromirror can be controlled by the voltage applied to the electrodes, expands. However, the voltage applied to the electrodes must be increased to obtain a desired angle of the micromirror, and the height of a sacrificial layer formed in manufacturing of a micromirror actuator must be increased.
To solve the above-described problems, it is an aspect of the present invention to provide a micromirror actuator, which is capable of obtaining a larger driving angle with the use of a lower driving voltage by forming a stepped electrode in a trench (thus lower than in FIGS. 1A and B) so as to make the difference between the lower electrode and a micromirror vary.
According to the present invention, a micromirror actuator comprises a substrate, spring units elastically supported by protrusions formed on the substrate, a micromirror connected to the spring units and capable of rotating, trenches formed in the substrate at either side of the protrusions to correspond to the surface of the micromirror and lower electrodes formed in each of the trenches. Lower electrodes may include an electrode formed at the bottom and sidewall of each of the trenches or between the trenches. Trenches may have vertical, slanted or stepped sidewalls. Further, in order to enlarge the range of the driving angle of the micromirror, the distance between the lower electrodes and the micromirror is varied as well as the distance between the lower electrodes and their size.